The Rare Books and Manuscripts Controlled Vocabulary defines commonplace books as "personal notebooks in which the owner has copied passages of interest or written his or her own compositions; use also for printed versions of such works."
Digitized commonplace books refers to physical commonplace books that have been captured (either through scanning or photography) and represented digitally. These digital representations are often available online, mostly through free, open-access platforms hosted by the owning institution. For example, the Folger Shakespeare Library has digitized many of its commonplace books and makes them available through their Folger Digital Collections website.
As you endeavor on creating your own commonplace book, take a moment to browse through the collections listed below for inspiration from the 17th century and more: